In Sunni Islam, today commemorates the day when the Israelites were freed from Egypt and the Pharaoh. It is an optional day of fasting. For Shi'a Muslims, today is a day of mourning for the martyrdom of Husayn ibn Ali, the Prophet Muhammad's grand some, at the Battle of Karbala in AH 61 (680 CE). In several countries, it is a national holiday and a significant day of public mourning and remembrance. The Wikipedia site provides more information.